The invention relates generally to compressor systems and, more particularly, to lubrication of one or more bearings in a compressor of a refrigeration system. Substitute
Refrigerant systems are utilized in many applications to condition an environment. The cooling or heating load of the environment may vary with ambient conditions, occupancy level, other changes in sensible and latent load demands, and as the temperature and/or humidity set points are adjusted by an occupant of the environment.
Use of a variable speed drive for the compressor motor improves the efficiency of refrigerant systems. Often, the compressor need not be operated at full speed, such as when the cooling load on the refrigerant system is relatively low. Under such circumstances, it might be desirable to reduce the compressor speed, and thus reduce the overall energy consumption of the refrigerant system. Implementation of a variable speed drive is one of the most efficient techniques to enhance system performance and to reduce life-cycle cost of the equipment over a wide spectrum of operating environments and potential applications, especially at part-load conditions.
However, compelling reliability concerns limit the allowable compressor speed reduction. In particular, inadequate lubrication of the compressor elements such as bearings may present a problem at low operating speeds. Speed dependent reliability concerns arise because damaging contact may occur between two surfaces in close proximity depending on their relative speed and the viscosity of the lubricant between them. As the speed is reduced, the viscosity of the lubricant must be increased to maintain a separating film between the two surfaces. The viscosity required to maintain separation of the two surfaces at a given operating speed depends in part on geometric features of the surfaces, so different types of bearings may have different requirements to prevent damage from occurring. Commonly, bearings have a smaller diameter have a higher viscosity requirement at any speed, but geometric features may override such diameter effects.
Most oils used in refrigerant screw compressors form a solution of mixed refrigerant and oil. When mixed, the refrigerant dilutes the oil, lowering the viscosity of the resultant refrigerant-oil mixture compared to the viscosity of pure oil. The amount of refrigerant dissolved in oil in a stable solution is a chemically determined function of pressure and temperature. During non-equilibrium transients, such as may occur during pressure drop just downstream of an orifice, or due to heat addition, or due to mechanical action that induces cavitation, refrigerant can out-gas from the solution as a new equilibrium state develops. Such occurrences of out-gassing generally increase viscosity because they result in less dilution of oil.
Bearing operation introduces viscous losses that result in heating of the lubricant. Heat transfer from hot portions of a compressor housing may also raise lubricant temperature. The resulting increase in lubricant temperature my cause out-gassing of some refrigerant. In addition, mechanical agitation of the lubricant as it passes through bearings can also cause cavitation which results in refrigerant out-gassing. As a result of out-gassing, lubricant flow exiting bearings usually has higher viscosity than when it entered bearings because the fraction of refrigerant in solution has been reduced.
Due to the minimum speed limitation that must be imposed to ensure reliability, some of the energy efficiency that could be potentially provided by the variable speed drive is essentially eliminated. Thus, there is a need to provide a compressor that can reliably operate at a lower speed than what can be achieved with current designs.